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From rats to bedbugs, how super pests are surviving and thriving in cities like Toronto

A new study shows that when human urbanization meets animal evolution, human health can suffer.

One of the studies reviewed by University of Toronto researchers focused on brown rats in a park in the Paris suburbs. It showed more than half are genetically resistant to rat poisons and carry parasites, some of which are transmissible to humans.

PHILIPPE LOPEZ / AFP

One of the studies reviewed by University of Toronto researchers focused on brown rats in a park in the Paris suburbs. It showed more than half are genetically resistant to rat poisons and carry parasites, some of which are transmissible to humans.

Ever wonder why that spray isn’t getting rid of your bedbugs or why those pigeons in the park aren’t afraid of you?

Science might have the answer.

A new study review from the University of Toronto looks at how humans, as they build cities, are forcing the evolution of highly adjusted super pests often with harmful results.

“There are clear examples of where evolution in organisms seems to be influencing the evolution of human health,” said co-author Marc Johnson, associate professor of biology at U of T Mississauga and director at the university's Centre for Urban Environments.

He worked with Jason Munshi-South, associate professor of biological sciences at Fordham University, to examine 192 studies. Their review, just published in the journal Science, shows that 134 species around the world appear to have had their evolution knocked off-kilter by human action.

“And that’s just where we have direct evidence,” Johnson said.

The review took a year to complete and covered birds, insects, plants, reptiles and more, across various types of evolution. The results startled the researchers.

“Bedbugs, mosquitos, rats, cockroaches and pubic lice are all showing adaptations,” Johnson said. One example: bedbugs have become resistant to pyrethroid insecticides.

“When I was growing up, bedbugs weren’t an issue,” he said. “In the last two decades they’ve started popping up all over the place. Those old insecticides? They don’t work.”

The pair’s research uncovered the story of how, when people sheltered from bombs in the London Underground during World War II, they were plagued by a very different type of mosquito.

“They didn’t need a dormancy period, and they didn’t need a blood meal,” Johnson said. “They’ll take one but they don’t need it to produce their eggs. Research showed they had become genetically differentiated from the ones that lived above ground.”

Strikingly, when the below-ground mosquitoes were introduced to their above-ground cousins, they didn’t even recognize each other and wouldn’t mate. The strain has since spread to other major cities, and possibly Toronto, Johnson said.

A rat surge distressed locals and made headlines in Oshawa last month, and the number of rats combing Toronto has reportedly spiked in recent years, including a notable 2015 infestation at St. James Park. It comes as no surprise to those who have studied the ability of rodents to change and thrive.

“There is evidence that rats adapt to warfarin, the poison used to control them,” Johnson said, adding that their review showed how rats even vary genetically in Manhattan along a specific north-south divide.

Yet just as the super pests might be bad for our health, it’s equally amazing that some species crucial to our ecosystems have been able to handle everything we have thrown at them, Johnson said.

“For most of my life I’ve been trying to get out of cities,” he said. “But there’s actually amazing biology all around us.”

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